about this fan controller?

i'm trying to analyze this circuit, the one that is attached...does anyone know how diode sensors work? i mean in the circuit...and is the uA741 IC use as an amplifier? and how does the output of the 741 able to switch Q1...i just want to know how the system works...thank you very much!!!

All silicon based semiconductors have 2mV/C-grad change of forward voltage.